Advanced safety is required for a nuclear power plant, and therefore, in addition to a control system for controlling a normal operation of the plant, another control system, which is called a safety protection system, is provided thereto. In order to ensure the safety of the plant, extremely high reliability is required for the safety protection system so that the operation of a reactor shutdown system and an engineered safety features can be automatically started.
In recent years, the safety protection system is configured by using a digital control device which is multifunctional and flexibly controllable. The digital control device to be used in the safety protection system has a stand-by redundant constitution which includes arithmetic units one of which constitutes a currently used system, and the other arithmetic unit constitutes a stand- by system in order to realize the high reliability (for example, refer to Patent Literature 1). Then, the arithmetic unit, which constitutes the stand-by system, performs a tracking by itself so that the data to be processed always corresponds to the data of the currently used system, thereby capable of immediately taking over the process from the arithmetic unit of the currently used system when an abnormal event has occurred in the currently used system (for example, refer to Patent Literature 2).